Reveling in warmth, hoping for cold

I enjoy mild winter weather as much as the next person. It’s so refreshing to emerge from the house on an unseasonably warm and dry day and bask in the sun’s rays while catching up on garden chores. 

People tend to think of cold weather as inconvenient at best and damaging at worst. Frigid days and nights lead to chilly hands and feet, higher heating bills, and other nuisances. In our gardens, predictions of overnight frost or deep freezes mean that we need to protect our cold-sensitive citrus trees, tender perennial plants, and outdoor pipes from damage.

Cold days might be unwelcome, but the horticulturalist in me yearns for more of them. Why, you might ask?

Cold temperatures are an essential element of the Northern California climate, and they’re actually beneficial to our native plants, many favorite perennials, and orchards. The natural yearly progression from warm weather to cold and back again triggers biochemical responses in plants that regulate their growth cycles. Cold weather is one signal that plants heed to begin their winter rest period, called “dormancy.” Like people, plants need their sleep.

The dormant period begins in the fall when the day length shortens and temperatures decrease. These changes prompt deciduous plants to drop their leaves and produce growth-inhibiting hormones. Those hormones prevent the plants from “leafing out” during the winter, even if there are periods of unseasonably warm weather. This chemical mechanism protects the plants by delaying the growth of tender new leaves, which would be damaged if the weather suddenly turned cold again. The dormant period is broken only when the plant experiences a cold spell of sufficient length to break down the hormones. This process is often referred to as chilling or vernalization. 

Such chilling is also essential to our local agricultural production. Temperate fruit and nut trees — those species that go dormant in winter but can’t survive extreme cold — need a specific, cumulative number of chill hours where temperatures are between 32 and 45 degrees Fahrenheit. Without adequate and consistent cold weather, their productivity can suffer tremendously: leaf bud growth and development will be delayed, flower buds will drop or be poorly formed, flowering can be prolonged (thus making blooms more susceptible to diseases), and fruit set will be reduced.

Some common orchard trees have high chill requirements. Depending on the variety, walnut trees need 500 to 700 chill hours to break dormancy. The ever-popular Bing cherry needs 900 chill hours to effectively bloom and set fruit. And the Bartlett pear, which comprises roughly 75% of the world’s pear production, needs a whopping 1500 chill hours each winter! On the other hand, almonds, figs, olives, pecans, and persimmons have relatively low chill needs. To read more about this topic, see the University of California’s online publication, The California Backyard Orchard, “Tree Selection” (

A Manteca almond orchard in full and glorious bloom. (Photo by Kathy Ikeda)

Chill hours are one set of data recorded throughout the state by the California Irrigation Management Information System (CIMIS). Our local CIMIS station is located in Manteca, and during the 2018-2019 winter season — measured from the first day of November to the last day in February— it recorded 924 chill hours. During the unseasonably warm winter of 2014-2015, we had only 723 chill hours.

As of February 9 this year, San Joaquin County has an accumulated winter total of 881 chill hours. That might sound adequate, but other factors come into play as well. When cold weather is interrupted by periods of several consecutive days of warm, sunny weather, the cumulative seasonal chilling requirement can increase. Spring-like weather during winter essentially offsets some of the prior chill hours. According to the Master Gardener Handbook, “Cloudy or foggy weather that maintains temperatures below about 60°F during the day and 45°F at night is often necessary in parts of California to achieve adequate chilling hours.”

For the sake of our plants, we should rejoice in a normal season of cold winter weather.

With the certainty of warming trends due to climate change, home orchardists might want to plan ahead when selecting new fruit or nut trees. Consider planting types with naturally low chill needs, or buy fruit varieties specifically bred to have “low chill” requirements — 300 hour or less of temperatures below 45 degrees F. (Low-chill cherry cultivars have only been developed in the last couple decades.)

If we do get more near- to below-freezing temperatures this year, here are a few tips on caring for cold-tender plants:

  • Move sensitive potted plants indoors or to a protected area (under a patio cover or overhang, or against a wall that’s warmed by the sun).
  • Keep in-ground and potted plants well watered, because dry plants are more susceptible to cold damage.
  • Drape old-style Christmas lights over citrus trees and other tender plants. Unlike the newer styles of bulbs, old incandescent bulbs generate enough heat to provide a measure of protection from the cold.
  • If plants are damaged by frost, don’t remove any dead or dying growth until the risk of freezing weather is past, because the damaged leaves and stems will help insulate and protect the still healthy parts of the plant.
  • Once it’s safe, prune away all damaged parts. Dead growth will be spongy or limp, and if the bark is gently scraped away from a part of a dead stem, the color below will be black or brown. On the other hand, living tissue will be firm, and a thin layer of green will appear below the bark.
  • Avoid pruning live plants too early or heavily, because that could stimulate them to produce new, cold-sensitive growth. 

For gardening-related questions, call the UC Master Gardener office at 209-953-6112, or visit our website

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